Two newly isolated proto-oncogenes, erbB-2 and arg have been localized by in situ hybridization to chromosomes 17 and 1, respectively. The erbB-2 gene is frequently overexpressed in mammary cancer and the long arm of chromosome 1, where arg gene is located, is involved in structural rearrangements or duplications in the majority of the solid tumors. Human cytochrome P1-450 was localized on chromosome 15 at the site of breakpoint of the translocation 15;17 characteristic for acute promyelocytic leukemia. A North American Burkitt's lymphoma cell line exhibits, in addition to a common 8;22 reciprocal translocation, two other translocations occurring near proto- oncogene sites; however, only myc mRNA is highly elevated, indicating that myc gene was implicated in neoplastic development of this B-cell malignancy.